*36c3 preroll music*
Herald: So who is excited about photography or videography?
*applaus* Herald: Yeah? The title of the talk kind
of gives us gives it away. OK. We bet we are waiting for the last people to come in
and take a seat. Last time, raise your hands if you have a free seat next to you.
Every one of you coming in, look for raised hands and take your seat and then we will start.
Yeah, very good. OK. Looks like the doors are finally closed. Okay, so the next talk
on the second day is about ultrafast imaging. So many of you have done
videography or photography. Have thought about exposure time, how fast you can do
your photography. And some of your might have played with lasers and have built
blinky stuff with it or have done scientific experiments and Caroline Will
now show us what happens if we take those to combine them and take it to the
extreme. Caroline is working at DESY since four years. She has not done her PhD and
is now working in a group for theoretical fast modeling of inner workings of
molecules and atoms. She is doing a computational work and working together
with experimentalists to verify their observations, and now she is presenting
the inner mechanics of what she is doing and how we can actually maybe photograph
molecules by their forming. Applause!
*applause*
Caroline: Great. Yeah. Thank you very much for the introduction and thank you very
much for having me here. I'm excited to see this room so full. So I'm going to
speak today about an ultrashort history of ultrafast imaging. It's a really broad
topic. And I'm just gonna present some highlights, some background. Before I
start, I'd like to give you a few more few more words about myself. As we've already
heard, I work at DESY, this is the DESY campus you see here and in the Center for
Free Electron Laser Science, circle in orange. That's where I did my PhD. So this
whole campus is located in Hamburg. This is probably also a familiar place to many
of you. And now this year we are in Leipzig a bit further away for the 36th
Congress. So I'd like to start with a very broad question. What is the goal of
ultrafast imaging? And we've heard already that ultrafast imaging is related to
photography. Now, as many of you know, when you take a picture, with a quite long
exposure time, you see just a blurry image, for example, in this picture of a
bowl of water. We can hardly see anything. It looks a bit foggy. But if we choose the
correct exposure time, which in this case is 100 times shorter in the right picture
than in the left picture, then we see a clear image and we can see dynamics
unfold. So we have here, a drop of water that is bouncing back from the bowl and
also some ripples that are forming on the surface of this bowl. This is only visible
because we chose the right exposure time. And this is to me really the key of being
successful in ultrafast imaging to take a clear picture of an object that is moving.
But it's not enough to say take just a picture. So now imagine you're a sports
reporter. You get these two pictures and you're supposed to write up what happened.
So it's complicated. So the top picture is the start, the bottom pictures is the end.
Just from these two pictures, it's hard to see. But if we see before picture, we can
see very complex dynamics unfold. There are particles accelerating at high
velocity *laughing* coming in from the back. And even particles we did not see in the first
picture at all somehow are very relevant to our motion. And not only skiing races
are very dynamic, but most processes in nature are also not static. This is true
for everything we see around ourselves, but it's especially true for everything
that is quite small in the microcosm. And in general, we can gain a lot more insight
from time resolved images. So from ultra short movies. I'd like to show you the
very first ultrafast movie that was ever taken. Or maybe even the first movie that
was taken at all. This guy, Eadweard Muybridge lived in the 19th century. And
very shortly after the invention of a photography method, he tried to answer the
question does a galloping horse ever lift all of its feet off the ground? Why, it's
running. To us, it may seem like not so important question, but in the 19th
century, the horse was the main method of transportation, and horse races were very
popular. So there was a lot of interest in studying the dynamics of a horse, and this
process is too fast to see with the naked eye. But Muybridge implemented a stop
motion technique where the horse as it is running, cuts some wires, that then
trigger photographs. And with this he was able to take these twelve photographs of
the horse in motion. That was published under this title in Stanford in the 19th
century. And we see very clearly in the top row third picture and maybe also
second picture that indeed the horse lifts all of its legs off the ground, which was
a new insight at that time. And when we stitch all of these snapshots together, we
have an ultra fast movie of a horse galloping, which might be seen as the
first movie that was ever made in the history of mankind. Now, when I say
ultrafast today, I'm no longer thinking about horses, but about smaller things and
faster things. But let's go there, very gently. So the time scale that we are all
familiar with that we can see with the naked eye is something of the order of
seconds. So, for example, the acceleration of this cheetah, we can see with the naked
eye. Now, if we zoom in on this motion, we see that there are muscles inside of the
animal that are contracting as it is running. And this muscle contraction takes
place within milliseconds. So that's a part of a thousand in one second. But we
can go even smaller than that to the microsecond. So proteins inside of the
muscles or in any biologic matter fold and unfold on a timescale of microseconds.
That's already a part in a million of a second. Now going even smaller, to
nanoseconds there's certain dynamics that take place within these proteins, for
example, of how they dissolve in water. But the timescale that I'm interested in
today is the femtosecond. It's even faster than that it's the timescale where
individual atoms move in molecules as shown in this animation. Now a
femtosecond is very short. It's a part in a million of a billion of a second, or as
we physicists like to call it, ten to the minus 15 seconds because it's easier to
spell *laughing* to us. We can - to us - , we can go even faster than that. The time scale of
electronic motion and in molecules would be an attosecond. I'm just mentioning it
here because we don't stop at molecules, but nature is even faster than that. But
for the purpose of this talk, I will mainly focus on processes that take place
within the femtosecond. So within ten to the minus fifteen seconds. Now, this time
scale is something that is not really related to what we think about in everyday
life. But there are certain processes in chemistry, biology and physics that are
really fundamental and that start at this time scale. Just to give you an idea how
short a femtosecond is, the width of a human hair is about 100 micrometer. It's
shown here in an electron microscopic picture. And for light at the speed of
light, it takes only thirty femtoseconds to cross the hair. So that's how fast a
femtosecond is. And even although this timescale is so short, there are many
important processes that start here, I'd like to mention, just two of them. The
first one is vision in our eyes and our retina there sits a molecule called
rhodopsin, that is shown here to the left. And when light hits rhodopsin, it starts
to isomorphise, which is a fancy word for saying it changes its shape. And this
transmits, in the end, electrical impulses to our brain, which enables us to see. And
this very first step of vision takes only two hundred femtoseconds to complete. But
without it, vision would not be possible. Another very fast process that is
fundamental in nature is photosynthesis, where plants take light and CO2 and
convert it to other things, among them oxygen. And the very first excitation
where light hits the plant and it starts to make all this energy available. That
also takes less than one hundredth femtoseconds to complete. So really the
fundamental questions of life lie at this timescale. And I'd like to just mention
that all of these processes are not only very fast, but they also take place in
very small objects, that are of a size of a few atoms to nanometers, which makes it
also hard to observe because we cannot see them with the naked eye or with standard
microscopes. Now, we've seen already that it's important to choose the right
exposure time to get a clear image of something that's moving, but the kind of
method that we need for taking such a photograph of something that is moving
depends a lot on the timescale. So for stuff that is moving within seconds or
fractions of a second, we can see that with the naked eye, we can use cameras to
resolve faster motion, very much like Muybridge did with the very first camera.
Today, of course, we can go much faster to maybe a few microseconds. With very fancy
cameras called opto-electronic street cameras - i won't go into detail here - we
can go down to picoseconds. So we are already very close to the motion of
molecules, but we are not quite there yet. The timescale that we want to investigate
is a femtosecond. So really a time timescale of molecular motion and
electronics are not fast enough to reach this timescale. So we need something new.
And fortunately, we can create light pulses that serve as to say flashes, but
take snapshots of our moving molecules with femtosecond time resolution and light
pulses can be made so short. So in the following, I'm going to show you a bit
more detail on how we can use these ultra short light pulses to take snapshots of
moving molecules. The first method that I would like to briefly show you is X-Ray
diffraction, where we have an ultra short pulse, an X-Ray pulse coming in. It hits a
sample shown here in the red bubbles. That's essentially a molecule that that we
just place in the beam and it produces a so-called diffraction pattern that we can
then record on a screen. Now, the whole process is quite complicated. So I like to
just sketch the very basics of it. We see here X-Ray radiation hitting a crystalline
sample here to the left and the sample is excited, starts to radiate X-Ray back and
on the right we can see the X-Rays leaving the sample again. They will interfere and
we can record this pattern on the screen. So this is what we see here in this
visualization to the right. With this, we can feed a reconstructionalgorithm that
allows us to transform back our diffraction pattern that we've seen here
for for in this case a bio molecule. We can reconstruct from that the image as it
was in real space. So this is some protein, I believe. X-ray diffraction is
very nice for resolving small structures with atomic detail. Another method how we
can take snapshots using ultra short pulses, that I would like to briefly
introduce is absorption spectroscopy. Now you may know that light contains several
colors. For example, you've surely have held a prism in hand, and the prism can
break white light up into all the colors of a rainbow, that we can see with the
eye. Now we can do the same with X-Ray pulses. Then we cannot see the colors
anymore. So just let's just stick with a prism here. When we place a molecule in
front of all these colors, the molecule will block certain colors. That's quantum
mechanics. You just have to believe it or learn about it in long studies. So the
molecule is placed in front of all these colors. And to be right, the absorption
spectrum is recorded and the parts of the spectrum that are very bright correspond
to the colors that have been blocked by the molecule. And this is a very nice
technique to investigate ultra short dynamics, because where these lines are
located is characteristic of the chemical elements that we find in the molecule. For
example, if we use X-Ray radiation for this specific molecule, that I've shown
here lysine, that's not so important which molecule it is. We have three different
atoms in this molecule that are important carbon, nitrogen and oxygen and they
absorb at very different colors so we can keep them apart when we take the spectrum.
But not only that, we can take the spectrum at a later time when the molecule
has moved around a bit and we will see that the colors, the position of the lines
have changed a tiny bit. So it's really not much and I accelerated it already in
this visualization quite a bit. But with experimental methods, we can resolve this.
And this allows us to then trace back to how the molecule was moving in between
when we took these two snapshots. There are many more methods that you can use to
take ultrafast images. So we call them probe signals because we probe the
ultrafast motion of a molecule with such an ultra short pulse. For example, we can
record photo electrons or we can record fragments of a molecule and many more. But
I won't go into further detail here because this is not an exhaustive list of
methods that we can use. I'd rather like to show you how we can take molecular
movies so how we can combine all these ultrashort pulses to in the end film a
molecule in action. Now we've already seen in the movie of the horse that we need to
stitch several snapshots together and then we have a full picture, full motion of a
molecule. So we just like to do the same, but ten to the 15 times faster, should not
be too difficult, right? So we use our ultra short pulse. First ultrasound parts
that we use as a trigger, parts that sets off the motion and the molecule. This
defines us a certain time zero in our experiment and makes it sort of repeatable
because we always start the same kind of motion by giving it a small hit and now
it's just moving around. So we wait for a certain time, a time delay and then come
in with a probe pulse. The probe pulse takes a snapshot of a molecule. This goes
to some detector, goes to a kind of complicated reconstruction method that we
just execute from our screen. And with this, we reconstruct a snapshot of a
molecule. But this is only one snapshot and we want a whole movie. So we need to
repeat this process over and over again by shining and more and more probe pulses.
And this will create more and more snapshots of a molecule. And in the end,
we could stitch all of these together and we would arrive at the same image that you
see in the in the middle where the molecules is happily moving around. There
is one little problem: The probe pulse typically destroys the molecule. This is
very different. This is very different from taking pictures of a horse. The horse
normally survives. *laughting* So the probe pulse destroys the molecule. It just goes away.
So for each of these snapshots we need to use a new molecule. So we typically have a
stream of samples that is falling from the top to the bottom in our experiment. And
then we have to carefully align two pulses a trigger pulse and a probe pulse that
come together and take a snapshot of this molecule. And of course, we have to find a
method on how to make identical molecules available in - Yeah - you see, there's a
lot of complications with doing these experiments that I'm completely leaving
out here. So now we want to take a molecular movie and we know that we want
to have ultra short pulses to do so. But I didn't tell you yet what kind of light
source we need. So there are many light sources all around us. We have here lights
from lamps. I have a light in my laser pointer with light from the sun. But we
need quite specific light sources to take these snapshots of molecular motion. We've
already established that we want ultrashort pulses because else we cannot
resolve femtosecond dynamics, but for the proper kind of wavelength that we need I
would like to quickly remind you of the electromagnetic spectrum that you've
probably seen at some point in high school. So, so light, as you see here in
the bottom picture is an electromagnetic wave that comes in different wavelengths.
They can be quite long as in the case of radio waves to the very left. Then we have
the region of visible light shown here as the rainbow that we can perceive with our
eyes. And then we have wavelengths that are too short to see with our eyes. First,
UV radiation, that gives us a tan in the summer if we leave our house and then we
have X-ray radiation, soft and hard X-ray radiation that have atomic wavelength. So
the wavelength is really on the order of the size of an atom. So what kind of
wavelength do we need to study ultra short dynamics - ultra fast dynamics? We can
first think about what kind of wavelength we need when we want to construct an ultra
short pulse. I've drawn here two pulses to the left, a slightly longer pulse to the
right, a shorter pulse. And now if you think about squeezing the left parts
together such that it becomes shorter and shorter, you see visually that the
wavelength also needs to shrink. So we need shorter wavelengths for the shorter
the pulse we want to make. So this will be located somewhere here in this region of
the electromagnetic spectrum. And another important thing that we need to keep in
mind is if we want to take pictures by X-ray diffraction, we are limited, so we
can only resolve structures that are about the same size as the wavelength we used to
take our diffraction image. So if we want to take a picture of something with atomic
resolution, our wavelength needs to be of atomic size as well. And this places us in
the region of X-Rays drawn here, that have a wavelength of less than a nanometer. So
we can establish that we want small wavelengths in general. We have two
additional requirements that would just touch upon very briefly. First, we need
very brilliant pulses because the pulses are so short, we need to have a lot of
light in the short pulse. You can think about taking a picture in a dark room with
a bad camera. You won't see anything. So we need very bright flashes of light.
Another requirement is we need coherent laser light. So we cannot just use any
light, but it needs to have certain properties like laser light.
Unfortunately, the lasers that you can buy commercially do not operate in the region
of the electromagnetic spectrum that we are interested in. So we need to come up
with something new. And I will show you how we can generate ultra short pulses
both in the laboratory where we can generate pulses that are very short and
extend up to maybe the soft X-ray region. And another method to generate ultra short
pulses is at free electron laser sources, where we can go really to the hard X-Ray
regime. But first I'd like to go to the laboratory. So in the laboratory, it's
possible to generate an ultrashort pulse by using a process that's called high
harmonic generation. In high harmonic generation we start off of a high
intensity pulse, that's a red pulse coming in from the left, which which is focused
in a gas cell. And from there, it generates new frequencies of light. So the
light that comes out is no longer red, but it's violet, blue. We cannot see it with
the naked eye. So that's an artist's impression of how high harmonic generation
works. Before going into more detail about why this method is so good at producing
ultra short pulses, I'd like to mention that this is only possible because we have
the high intensity driving pulses, the red laser pulses available. This goes back to
work by Donna Strickland and Gerard Mourou, who were awarded the Nobel Prize
in the year 2018 in physics for this work that has been done in the 80s. Now we're
coming to the only equation of his talk, which is this equation that relates the
energy width and the time duration of a ultra short pulse. By the law of fourier
limits we cannot have pulses that are very short in time and at the same time very
narrow in energy. But we need to choose one. So if we want to have policies that
are very short in time like the pulse that I've shown here on the bottom, that is
actually only two hundred fifty attoseconds long, so even shorter than a
femtosecond, then we need to have a very broad width in energy. And this means
combining a lot of different colors inside of this pulse. And this is what makes high
harmonic generation so efficient at creating ultra short pulses, because the
spectrum that the colors that come out of high harmonic generation are shown here
and they really span a long width. So we get a lot of different colors with about
the same intensity. And you can think of it like putting them all back together
into one attosecond pulse. That is very short in time. This method has really made
a big breakthrough in the generation of ultra short laser pulses we see here a
plot of a time duration of laser pulses versus the year, and we see that since the
invention of the laser, here in the mid 60s, there was a first technological
progress and shorter and shorter pulses could be generated. But then in the 80s,
there was a limit that had been reached of about five femtoseconds, I believe. And we
could not really go farther than that and only with high harmonic generation, that
sets in here shortly before the year 2000, we were able to generate pulses that are
of a femtosecond duration. So that really touch the timescale of molecular motion.
The current world record is a pulse, that is only 43 attoseconds long, established
in the year 2017. So that's really the timescale of electrons and we can do all
sorts of nice experiments with it where we directly observe electronic motion in
atoms and molecules. This is all very nice, but it has one limitation: We cannot
go to hard X-rays, at least not right now. So high harmonic generation cannot produce
the kind of very short wavelengths that we need in order to to do X-ray diffraction
experiments with atomic resolution. So if we want to have ultra short pulses that
have X-Ray wavelengths, we need to build right now very complex, very big machines,
the so-called free electon lasers. Now, this would be a specific light source that
can produce ultra short pulses with X-ray wavelengths in itself. The X-Ray
wavelengths is not so new. We know how to take X-ray images for about one hundred
and thirty years and already in the 50s. Rosalind Franklin, who is looking at a
microscope here, was able to take a picture of DNA, an X-ray diffraction
pattern of a DNA double helix that was successful in revealing the double helix
structure of our genetic code. But this is not a time resolved measurement. So think
of it as you have a molecule that is in crystalline form, so it's not moving
around and we can just take an X-ray image of it, it's not going anywhere. But if we
want - if we want to take a picture of something that is moving, we need to have
very short pulses. But we still need the same number of what we call photons, light
particles. Or think of it as we need more brilliant X-ray flashes of light than we
could obtain before. And there was very nice technological development in the past
50 years or so, where we were able to go from the X-ray tube to newer light sources
called Synchrotron, and today, free electro lasers that always increase the
peak brilliance in an exponential way. So we can take really brilliant, really
bright X-ray flashes right now. I cannot go into the details of all of that, but I
found a very nice talk from two years ago, but actually explains everything from
Synchrotron to FELs still available online if you're interested in this work. And as
always, if something is failing scaling exponentially, most of you will be
familiar with Moore's Law, that tells us about the exponential scaling of
transistors. If something grows this fast, it really opens up a new series of
experiments of new technological applications that no one has thought of
before. And the same is true with free electron lasers. So I'm going to focus
just on the most brilliant light sources for X-rays. Right now, the free electron
lasers that are at the top right here of this graph have been around for maybe 10
years or so. I cannot go into a lot of detail on how to generate ultrashort
pulses with X-Rays. So I'd like to give you just a very broad picture of how
this works. First, we need a bunch of electrons, that is accelerated to
relativistic speed. This sounds very easy, but is actually part of a two kilometer
long accelerator, that we have to build and maintain. Now we have this bunch here
of electrons shown in red and it's really fast and now we can bring it into
something that is called an undulator. That's a series of alternating magnets,
shown here in green on blue for the alternating magnets. And you may remember,
that when we put an electron, that is as a charged particle, into a magnetic field,
the Lorence force will drive it away. And if you have alternating magnets, then the
electron will go on a sort of wiggly path in this undulator. And the electron is a
charged particle as it is wiggling around wherever it turns around, it will emit
radiation, that happens to be in the X-ray region of the electromagnetic spectrum,
which is exactly what we want. We can watch this little movie here to see a
better picture. So this is the undulating seeing from the side. We now go inside of
the undulator. We have a series of alternating magnets. Now the electron
bunch shows up and you see the wiggly motion as it passes the different magnets.
And you see the bright X-Ray flash that is formed and gets stronger and stronger as
the electron bunch passes the undulator. So we need several of these magnet pairs to
in the end, get the very bright X-Ray flash. And at the end of the undulator we
dump the electron, we don't really need this electron bunch anymore and continue
with a very bright X-Ray flash. This whole process is a bit stochastic in nature, but
it's amplifying itself in because of the undulator. This is why the longer the
undulator is, the more bright X-Ray flashes we can generate. This whole thing
is kind of complicated to build, it's a very complex machine. So right now there
are only very few free electron lasers in the world. First one in California called
LCLS 1, currently being upgraded to LCLS 2. There are several in Europe. There's
one in Switzerland, in Italy and Hamburg. So there's a Flash that does not operate
in the hot X-ray regime, but was kind of first free electron laser. That's the most
recent addition to the free electron laser zoo. It's the European XFEL also located
in Hamburg. And then we have some of these light sources in Asia, in Korea, South
Korea, Japan, and one currently under construction in Shanghai. I'd like to show
you a bit more details about the European X-ray free electron laser, because it's
closest to us, and at least closest to where I work. So the European XFEL is a
three point four kilometer long machine that is funded by in total 12 countries,
So Germany and Russia paying the most and then the other 10 countries also providing
to the construction and maintenance costs. This machine starts at the DESY campus,
but as shown here to the right of the picture. And then we have first an
accelerator line for the electrons that it's already one point seven kilometers
long and where we add electrons reach their relativistic speed. Then the
undulate comes in, so the range of magnets where we X-Ray flashes are produced. The x
X-Ray flashes then cross the border to Schleswig-Holstein, *laughter * shown here, on the
other side in a new federal state. They reach the experimental hall. We have in
total six experimental end stations at the European XFEL that provide different
instrumentation, depending on which kind of system you want to study, you need
slightly different instruments. And it's not only for taking molecular movies, but
the XFEL is used, among others, for material science, for the imaging of bio
molecules, for femtosecond chemistry, all sorts of things. So really wide range of
applications. It's right now the the fastest such light source can take twenty
seven thousand flashes per second, which is great because every flash is one
picture. So if we want to take a lot of snapshots, if you want to generate a lot
of data in a short time, it's great to have as many flashes per second as
possible. And as you can imagine, it's kind of expensive since there are so few
free electron lasers in the world to take measurements there. The complete price tag
for constructing this machine, it took eight years and cost one point two billion
euros, which may seem a lot, but it's the same amount that we spend on concert halls
in Hamburg. *loud laughter applause* So kind of comparable. Now, when you factor in maintenance and so on,
I think a minute of X-Ray beam at such an XFEL cost several thousands of tens of
thousands of euros in the end. So getting measurement time is complicated and there
are committees that select the most fruitful approaches and so on. So in order
to not to waste or do taxpayers money. With this, I'd like to make a small
comparison of the light sources that I've introduced now. So I introduced the
laboratory light sources and the XFEL light source. In general, in the
laboratory we can generate very short pulses of less than 100 attoseconds by now
and in the XFEL we are limited to something about 10 femtoseconds right now.
In terms of brilliance the XFELs can go to much more bright pulses, simply because
they are bigger machines and high harmonic generation in itself is a kind of
inefficient process. In terms of wavelength X-Ray free electron lasers
enable us to reach these very short wavelengths with X-Rays, that we need to
get atomic resolution of defractive images. In the laboratory we are a bit
more limited to maybe the soft X-ray region. There's another important thing to
keep in mind when we do experiments, that's the control of pulse parameters. So
is every pulse that comes out of my machine the same as the one that came out
of my machine before. And since the XFEL produces pulses by what is in the end, a
stochastic process, that's not really the case. So the control of possible
parameters is not really given. This is much better in the laboratory. And in
terms of cost and availability, it would of course, be nice if we could do more
experiments in the lab. Then at the XFEL simply because we XFEL ls so expensive to
build and maintain and we have so few of them in the world. And you can see this
tunnel here. It stretches for two kilometers or so, all packed with very
expensive equipment. So I'd like to show you a brief example of what we can learn
in ultrafast science. So this is a theoretical work that we did in our group.
So no experimental data, but still nice to see. This is concerned with an organic
solar cell. So we all know solar cells. They convert sunlight to electric energy
that we can use in our devices. The nice thing about organic solar cells is that
they are foldable, very lightweight, and we can produce them cheaply. The way that
such a solar cell works is we have light shining in and at the bottom of the solar
cell there sits an electrode that collects all the charges and creates an electric
current. Now light creates a charge that somehow needs to travel down there to this
electrode and in fact, many of these charges. So the important thing where we
build such an organic solar cell is that we need a way to efficiently transport
these charges. And we can do so by putting polymers inside. A polymer is just a
molecule that is made up of two different or two or more different smaller
molecules. And one such polymer, which should be very efficient at transporting
these charges is BT-1T, that is shown here of a name is not so important, it's an
abbreviation. Because in BT-1T when we create a charge at one end of a molecule
here at the top, it travels very quickly to the other side of a molecule and you
can imagine stacking several of these BT-1T or especially of the Ts together,
putting it in this material. And then we have a very efficient flow of energy in
our organic solar cell. So what we did was we calculated the ultrafast charge
migration in BT-1T, shown here to the right. The pink thing is the charge
density that was created by an initial ionization of the molecule. And now I show
you the movie, how this charge is moving around in a molecule so you can see
individual atoms moving, the whole molecules vibrating a bit. And the charge,
if you look closely, is locating on the right half of a molecule within about 250
femtoseconds. Now, we cannot observe this charge migration directly by looking at
this pink charge density that I've drawn here, because it's at least for us, not
experimentally observable directly. So we need an indirect measurement, an X-Ray
absorption spectroscopy that I showed you in the beginning could be such a
measurement. Because in the X-Ray absorption spectrum of BT-1T that I've
shown here in the bottom left, we see distinct peaks depending on where the
charge is located. Initially the charge is located at the top sulfur atom here and
this molecule and we will see a peek at this color. Once the charge moves away to
the bottom of a molecule to the other half, we will see a peak at the place
where nothing is right now because the charge is not there. But if I start this
movie, we will again see very fast charge transfer. So within about two hundred
femtoseconds, the charge goes from one end to the molecule to the other end of a
molecule. And it would be really nice to see this in action in the future XFEL
experiment. But the process is very long. You need to apply for time at an XFEL. You
need to evaluate all the data. So maybe a couple of years from now we will have the
data available. Right now we are stuck with this movie, that we calculated. Now,
towards the end of my talk, I'd like to go beyond the molecular movie. So I've shown
you now how to generate the light pulses and an example of what we can study with
these light pulses. But this is not all we can do: So when you think of a chemical
reaction, you might remember high school chemistry or something like this, which is
always foaming and exploding and nobody really knows what is going on. So a
chemical reaction quite naturally involves molecular dynamics, for example, the
decomposition of a molecule to go from here, from the left side to the right
side, the molecules will somehow need to rearrange so all the atoms will have moved
quite a bit. We've seen already how we can trigger these chemical reactions or these
molecular motion that was part of a molecular movie. But it would be really
cool if we could control the reaction with light. So the way to do this, it's not
currently something that is possible, but maybe in the near future, would be to
implement a sort of optimisation feedback loop. So we would record the fragments of
our reaction, send it to an optimization routine that will also be quite
complicated and will need to take into account the whole theory of how light and
matter, interact and so on. And this optimization routine would then generate a
new sequence of ultra short pulses and with this feedback loop, it might be
possible to find the right pulses to control chemical reactions, taking into
account the quantum nature of this motion and so on. Right now this is not possible.
First, because the whole process of how we can generate these ultra short pulses is
not so well controlled that we could actually implement it in such a loop. And
also the step optimization routine is more complex than it looks like here in this
picture. So this is something that people are working on at the moment, but this
would be something like the ultra fast wishlist for next Christmas, not this
Christmas. So we've succeeded in taking a molecular movie, but we would also like to
be able to direct a molecular movie. So to go beyond just watching nature, but
controlling nature because this is what humans like to best, * laughing* fortunately or
unfortunately, it depends. So I'd like to just show you that this is really an ultra
fast developing field. There's lots of new research papers every day, every week
coming in, studying all sorts of systems. When you just take a quick and dirty
metric of how important ultrafast science is this is the number of articles per year
that mentioned ultrafast in Google Scholar, it's exponentially growing. At
the same time, the number of total publications in Google Scholar is more or
less constant, so the blue line here outgrows the green line considerably since
about ten years. So what remains to be done? We've seen that we have light
sources available to generate ultra short pulses, but as always, when you have
better machines, bigger machines, you can take more fancy experiments. So it would
be really nice to develop both lab based sources and free electron laser sources so
that we can take more, more interesting, more complex experiments. Another
important challenge, that's what people in my research group where I work are working
on is to improve theoretical calculations because I did not go into a lot of detail
on how to calculate these things, but it's essentially quantum mechanics and quantum
mechanics skales very unfavorably. So going from a very small molecule like the
glycene molecule here to something like a protein is not doable. Simply, it cannot
compute this with quantum mechanics. So we need all sorts of new methodology to -
Yeah - to better describe larger systems. We would in general like to study not only
small molecules and not only take movies of small molecules, but really study large
systems like this is the FMO complex, that is a central in photosynthesis or solid
states that are here shown in this crystal structure simply because this is more
interesting for biological, chemical applications. And finally, as I've shown
you, it would be cool to directly control chemical reactions with light. So to find
a way how to replace this mess with a clean light pulse. With this, I'm at the
end. I'd like to quickly summarize femtodynamics, really fundamental in
biology, chemistry and physics. So more or less, the origin of life is on this
timescale. We can take molecular movies with ultrashort laser pulses and we can
generate these pulses in the laboratory or add free electron lasers with different
characteristics. And we would like to not only understand these ultra fast
phenomena, but we would also like to be able to control them in the future. With
this, I'd like to thank you for your attention and thank the supporting
institutions here that funded my PhD work.
*applause*
Herald: Well, that was an interesting talk. I enjoyed it very much. I guess this
will spark some questions. If you want to ask Caroline a question, please line up
behind the microphones. We have three in the isles between the seats if you want to
leave, please do so in the door here in the front. And until we get questions from
the audience, do we have questions from the Internet?
Signal angel: Yes. Big fat random user is curious about the design of the X-Ray
detector. Do you have any information on that?
Caroline: That's also very complex. I'm not a big expert in detectors. At this
point, I really recommend watching the talk from two years ago, that explains a
lot more about the X-Ray detectors. So what I know about the X-Ray detector is
that it's very complicated to process all the data because when you have 27000
flashes of light, it produces, I think terabytes of data within seconds and you
need to somehow be able to store them and analyze them. So there is also a lot of
technology involved in the design of these detectors.
Herald: Thank you. So the first question from microphone 2 in the middle.
Microphone 2: So my question is. Herald: Please go close to the microphone.
Microphone 2: My question is regarding the synchronization of the detector units when
you're pointing to free electron laser so you can achieve this in synchronization.
Caroline: This is also very complicated. It's easier to do in the lab. So you're
talking about the synchronization of essentially the first pulse and the second
pulse. Right. So in the lab, you typically generate the second pulse from part of the
first pulse. So you have a very natural alignment, at least in time of these two
pulses. The X-ray free electron lasers have special timing tools that allow you
to find out how much is the time delay between your two pulses. But it's true
that this is complicated to achieve and this limits the experimental time
resolution to something that is even larger than the time duration of the
pulses. Herald: So now next question from
microphone number 3. Microphone 3: Yes, i remember in the
beginning, you explained that your measuring method usually destroys your
molecules. That's a bit of a contradiction to your idea to control. *laughing*
Caroline: In principle, yes. But, so in the case of control, we would like to use
a second pulse, that does not destroy the molecule. But for example, at least
destroys it in a controlled way, for example. *loud laughter* So there's a difference between
just blowing up your molecule and breaking apart a certain part, but yet that we are
interested in. And that's what we would like to do in the control case. So we
would like to be able to to control, for example, the fragmentation of a molecule
such that we only get the important part out and everything else just goes away.
Microphone3: Thank you. Herald: So then another question for
microphone 2 in the middle. Microphone 2: So thank you for the talk. I
was interested in how large structures or molecules can you imagine with this lab
contributions and with this XFEL thing? Caroline: Sorry. Can you repeat?
Microphone 2: So how large molecules can you imagine in this laboratory with this
high harmonic measures? Caroline: So how large is not really the
fundamental problem? People have taken snapshots of viruses or bigger bio
molecules. If you want to - the problem is rather how small can we get? So yeah, to
take pictures of a very small molecule. Currently we cannot take a picture of an
individual small molecule, but what people do is they create crystals of a small
molecule, sticking several of them together and then taking images of this
whole crystal for single particles, I think right now about the scale of a virus
nanometers. Microphone 2: Thank you.
Herald: OK. Do we have another question from the Internet?
Signal Angel: We have. So this is concerning your permanent destruction of
forces, I guess. How do you isolate single atoms and molecules for analysing between
the different exposures? Caroline: Yes, excellent question. So
molecules can be made available in the gas phase by - so if you have them in a solid
somewhere and you heat that up, they will evaporate from that surface. This is how
you can get them in the gas phase. This, of course, assumes that you have a
molecule, that is actually stable in the gas space, which is not true for all
molecules. And then the, the hard thing is to align all three things. So the pump
pulse, the probe pulse and the molecule all need to be there at the same time.
There are people doing whole PhD theses on how to design gas nozzles that can provide
this stream of molecules. Signal Angel: So you basically really
having a stream coming from a nozzle? Caroline: Yes.
Signal Angel: It's a very thin stream, I guess.
Caroline: Yes. Signal Angel: Then you're exposing it like
in a regular interval. Caroline: And of course you try to hit as
many molecules as possible. So this is especially important when you do pictures
of crystallized molecules because crystallizing these molecules is a lot of
work. You don't want to waste like 99 percent that just fall away and you never
take snapshots of them. Signal Angel: Thanks.
Herald: So another question from microphone 3.
Microphone 3: How do you construct this movie? I mean, for every pulse to have a
new molecule and for every molecule is oriented differently in space and has
different oscillation modes. How do correlate them? I mean, in the movie, I
mean, every molecule is different than the previous one.
Caroline: Yes. Excellent question. That's, So first, what people can do is align
molecules. So especially molecules that are more or less linear. You can force
them to be oriented in a certain way. And then there's also a bit of a secret in the
trigger pulse that first sets off this motion. For example, if this trigger pulse
is a very strong proto ionization, then this will kill off any sorts of
vibrational states that you have had before in the molecule. So in this sense,
the trigger parts really defines a time zero, that should be reproducible for any
molecule that shows up in the stream and the rest is statistics.
Microphone 3: Thank you. Herald: So there's another question on
microphone 3. Microphone 3: Are there any pre pulses or
ghosts, You need to get rid of? Caroline: Sorry. Again.
Microphone 3: You have to control pre pulses or ghosts during this effect for
measurement. Caroline: That I'm not really sure of,
since I'm not really conducting experiments, but probably.
Herald: And another one from the middle, from microphone 2 please.
Microphone 2: I suppose if you apply for experimentation time at the XFEL laser,
you have to submit very detailed plans and time lines and everything. And you will
get the time window for your experiment, I guess. what's going to happen if you're
not completely finished within that time window? Are they easy possibilities to
extend the time or are they do they just say, well, you had your three weeks,
you're out apply in 2026? Caroline: Yeah, I think it's a regular
case, that you're not finished with your experiments by the time your beam time
ends. That's how it usually goes. It's also unfortunatly not free weeks, but it's
rather like 60 hours delivered in five shifts of twelve hours. So, yeah, you
write a very detailed proposal of what you would like to do. Submit it to a panel of
experts, both scientists and technicians. So they decide, is it interesting enough
from a scientific point of view and is it feasible from a technical point of view?
And then once you are there, you more or less set up your experiment and do as much
as you can. If you want to come back, you need to submit an additional proposal. So,
yeah, I think most experimental groups try to have several of these proposals running
at the same time, so that there is not a two year delay between your data
acquisition. But yes. No possibility to extend. It's booked already for the
complete next year. The schedule is fixed. Herald: So I don't see any more people
queuing up. If you want to pose a question, please do so now. In the
meantime, I would ask the signal angel if there's another question from the
Internet. Signal Angel: I have a question about the
dimensions of all those machines. The undulator seems to be rather long and
contain a lot of magnets. Do you have an idea how long it is and how many of those
electromagnets are in there? Caroline: Yeah. Sorry, I didn't mention
it. It's about, I think one hundred and seventy meters long in the case of the
European XFEL. I'm not sure about the dimension of the individual magnets, but
it's probably also in the hundreds of magnets, magnet pairs.
Herald: So is there more - excuse me, there is a question on the microphone
number 3. Microphone 3: Yeah. Hi. It's regarding the
harmonic light. Herald: Please go closer to the
microphone. Microphone 3: The harmonic light generator
that you were showing at the very beginning, just before the one that won
the Nobel Prize. And can you also produce light in the visible range? Or it has to
be in the visible range? Caroline: The high harmonic generation? So
in the in the visible range, you cannot create pulses that are so short that they
would be interesting for what I'm doing. The pulse that comes in is already quite
short. So it's already femtoseconds long. They just convert it into something that
is fractions of a femtosecond long. And yeah. In the indivisible range that's kind
of a limit how short your pulse can be. Microphone 3: So it is not a good
candidate for hyperspectral light source. We need another kind of technique, I
guess. Caroline: Well, I mean you are kind of
limited what short pulses you can generate with which wavelength.
Microphone 3: Thank you. Herald: So again, a question to the Signal
Angel. Are there more questions from the internet?
Signal Angel: Yes I have another one about the lifetime of the molecules in the beam?
How fast are they degrading or how fast are they destructed?
Caroline: So probably the question is about how fast they are destructed before
- so either before our pulses hit the molecule the molecules should be stable
enough to survive in the gas phase from the point where they are evaporated until
the point where the pump and the probe pulse come together, because otherwise it
doesn't make sense to study this molecule in the gas phase. When the probe pulse
hits and it flows apart, I guess pico seconds until the whole molecules ...
Microphone 3: Like instantaneous. Herald: So I don't see any more questions
on the microphones and we have a few minutes left. So if there are more
questions from the Internet, we can take maybe one or two more.
Signal Angel: Give me a second. Herald: For people leaving already, please
look if you have taken trash and bottles with you.
Signal Angel: So this one very, very technical question. How do you compensate
the electronic signal that the electronic signal reaction is probably slower than x
ray or light or spectrum changes at one moment or at one particular moment, that
was interesting to analyze. I do not understand the question, though.
Caroline: I think I understand the question, but I don't have the answer
because again, I'm a - that's the problem of speaking to a technical audience, you
get a lot of these very technical question. Yeah. The data analysis is not
instantaneous. So the data is transported somewhere safe in my imagination. And then
taken from there. So this data analysis does not have to take place on the same
timescale as the data acquisition, which I guess is also because of the problem that
was mentioned in the question. Herald: I might interrupt here. Maybe it's
also about the signal transmission, like the signal rising of the signal of the
electrical signal transmissions. Because this would probably require bandwidths of
several megahertz, gigahertz, I don't know, to transport these very fast
results. Caroline: Yeah. I think that's also a
problem of constructing the right detector. That has been solved apparently
because they can take these images. *laughter* Herald: And on the other hand, we have 10
gigabit either nets. So we get faster and faster electronics. More questions from
the Internet? Does not look like it, also the time is running out. So let's thank
Caroline for her marvelous talk. *Applause,* *final music*
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